In the description, soluble hydrated chromium oxide means the hydrated chromium oxide which is easily dissolved before drying into the fluoride bath or chromic acid solution without additives before drying and insoluble hydrated chromium oxide means the hydrated chromium oxide which is difficult to dissolve into the fluoride bath or chromic acid solution without additives even before drying.
Recently, the change from expensive electrotinplates to cheaper tin free steel having double layers consisting of a lower layer of metallic chromium and an upper layer of hydrated chromium oxide has rapidly taken place in the field of food and beverage cans, five gallon cans and miscellaneous cans. This is because the tin used for the production of tinplate is expensive and tin free steel has excellent lacquer adhesion compared with that of tinplate.
An ordinary metal can made of tin free steel consists of two can ends and a single can body, except for drawn can. In the case of tin free steel, the seaming of the can body is generally carried out with nylon adhesives by using the Toyo Seam (Trade name) and Mira Seam (Trade name) method. Another method of seaming a tin free steel can body by electric welding is also well known. In the case of the seaming of a tin free steel can body by electric welding such as the Soudronic process, however, the metallic chromium layer and the hydrated chromium oxide layer must be mechanically or chemically removed from the tin free steel surface in order to easily obtain a well seamed can body at high speed. Therefore the corrosion resistance in the welded part of the tin free steel can body becomes remarkably poor, even if this welded part is coated with lacquer after welding.
From the background described above, the development of a can material which is cheaper than tinplate and is easily weldable at high speed without the removal of the plated layer, has been required, especially in the field of food cans.
Recently, various methods for producing tin free steel which can be easily welded at high speed without the removal of the plated layer have been proposed. For instance, the methods shown in Japanese Patent Publication Nos. Sho 57-19752, Sho 57-36986 and Laid-Open Japanese Patent Application Nos. Sho 61-213398, Sho 63-186894 have been already known.
Japanese Patent Publication No. Sho 57-19752 relates to a tin free steel with excellent weldability having double layers consisting of a lower layer of metallic chromium of 3 to 40 mg/m.sup.2 and an upper layer of non-metallic chromium which is mainly chromium oxide of 2 to 15 mg/m.sup.2 as chromium. This Sho 57-19752 intends to improve the weldability of tin free steel by the formation of a porous metallic chromium layer with a small amount of metallic chromium. However, it is considered that not only the weldability but also corrosion resistance are poor, because the iron oxide film having high electric resistance is formed by the oxidation of the steel base deliberately exposed through the pore of metallic chromium layer during the lacquer curing.
Japanese Patent Publication No. Sho 57-36986 is characterized by the use of a chromic acid electrolyte with a small amount of anions such as sulfate ion, nitrate ion and chloride ion in order to produce a tin free steel with excellent weldability, formability and lacquerability which has metallic chromium of 0.5 to 30 mg/m.sup.2 and hydrated chromium oxide of 2 to 50 mg/m.sup.2 as chromium. This method for producing the tin free steel intends to improve the corrosion resistance, which becomes poor by a decrease in the amount of metallic chromium, by the improvement of the quality of the hydrated chromium oxide layer. However, the weldability of the tin free steel obtained by this method will be not improved because the exposed steel surface is oxidized by heating the lacquer coated tin free steel similar to Sho 57-19752 described above.
Laid-Open Japanese Patent Application No. Sho 61-213398 relates to a tin free steel for a welded can with excellent corrosion resistance after lacquering which has flatly deposited metallic chromium of 10 to 40 mg/m.sup.2 and uniformly formed hydrated chromium oxide of 3 to 30 mg/m.sup.2 as chromium. The method for producing this tin free steel is characterized by the dissolution of a part of the deposited metallic chromium by an anodic treatment after the formation of a double layer consisting of metallic chromium and hydrated chromium oxide. However, it is considered that the weldability of the tin free steel obtained by this method will be not improved because of the increase of the exposed steel base with the increase of the pore in the metallic chromium layer.
Laid-Open Japanese Patent Application No. Sho 63-186894 relates to a tin free steel for a welded can having metallic chromium of 50 to 150 mg/m.sup.2 and hydrated chromium oxide of 5 to 20 mg/m.sup.2 as chromium. The tin free steel obtained by Sho 63-186894 is characterized by the granular deposition of metallic chromium. This tin free steel was developed based on the fact that an electric contact resistance, which is used for one of the index of the evaluation of the weldability, became lower by the granular deposition of metallic chromium.
The weldability is usually evaluated by an available secondary current range in welding, that is, the wider the secondary current range in welding, the better the weldability. On determining the available secondary current range in welding, the upper limit corresponds to the welding conditions in which some defect such as splashing is found and the lower limit corresponds to the welding conditions in which the breakage occurs in the welded part by the tearing test. However, the weldability is usually evaluated by a simple method of electric contact resistance measurement, which has an apparent correlation to the available secondary current range in welding, because a large number of samples are necessary in order to determine the available secondary current range in welding.
Namely, the lower the electric contact resistance, the wider the available secondary current range in welding.
Although the electric contact resistance has an apparent correlation to the available secondary current range in welding for tin free steels having almost the same morphology of metallic chromium, this correlation is not recognized for tin free steels having the different morphology of metallic chromium. For example, in the case of a tin free steel having granularly deposited metallic chromium obtained by Sho 63-186894, the available secondary current range in welding is narrow in spite of the low electric contact resistance.
Therefore, the weldability of tin free steel is not evaluated by only the electric contact resistance, although the electric contact resistance is used as the index of the evaluation of the weldability for tin free steels having almost the same morphology of metallic chromium.
In general, there are two well-known types of method for producing an ordinary tin free steel. The first type is a one-step method in which metallic chromium and hydrated chromium oxide are formed in one operation by using one electrolyte composition. The second type is a two-step method in which metallic chromium is formed first by using one electrolyte composition as a chromium plating solution, and then hydrated chromium oxide is formed on the metallic chromium layer by using another electrolyte composition. In both types of processes, a chromic acid electrolyte with a fluoride compound (fluoride bath), with a sulfate compound (sulfate bath) or with both additives (mixed bath) are usually used. It has been known in the report by K. Yoshida et al. (Kinzoku Hyomen Gijutsu, vol. 30, No. 7, 1979, page 338) that the film in the ordinary tin free steel produced by the methods described above is constructed of three layers consisting of a bottom layer of metallic chromium, a middle layer of insoluble hydrated chromium oxide which is mainly equivalent to chromium oxide and an upper layer of soluble hydrated chromium oxide. Namely, the hydrated chromium oxide is constructed by two layers having different structures.